This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to steady state free precession (SSFP) imaging.
Magnetic resonance imaging (MRI) provides excellent soft tissue contrast with arbitrary scan-volume orientations, thus making MRI an extremely useful medical imaging modality. However, in many applications, MRI is limited by long scan times, limited spatial resolution, and contrast between lipid-based tissue and water-based tissue. Recent advances in gradient amplifier technology have enabled the use of fully-refocused steady-state free precession (SSFP) imaging methods. SSFP imaging is a very fast method that can provide good tissue contrast and high resolution. A number of commercial implementations of SSFP are available, all of which are conceptually identical.
As illustrated in FIG. 1, a refocused SSFP sequence consists of a single RF excitation which is repeated periodically. All gradients used for slice selection or imaging are fully rewound over each repetitive time, TR. In the steady-state, the magnetization at points a and d is the same. Magnetization is tipped about the x-axis through an angle α. Between excitations, the magnetization undergoes a precession by an angle θ=2πΔfTR about the z-axis, where f is the tissue off-resonance, and also experiences both T1 and T2 relaxation.
During the sequence each spin is affected by RF pulses, relaxation and free precession. The steady-state magnetization for SSFP is a function of the sequence parameters flip angle (a), repetition time (TR) and echo time (TE) as well as the tissue parameters T1, T2, and off-resonant frequency Δf.
Balanced SSFP imaging sequences require short repetition times to avoid off resonance artifacts. The use of slab-selective excitations is common, as this can improve imaging speed by limiting the field of view. However, the necessarily short duration excitations have poor slab profiles. This results in unusable slices at the slice edge due to significant flip angle variations or aliasing in the slab direction.